I think (and I emphasise think!) the B737 will be about 10 seats less than the A320 assuming they still pursue the extra legroom seating options to mirror the current offering. There are still lots of things being considered including keeping some Airbus for the routes that can sustain 200+ pax.
I think the new fleet will be a good opportunity to cut out any routes that don't offer enough profitability and expand those that do by upping frequency.
It will be very interesting to see what happens once the Max's do arrive. It must be quite exciting for those involved particularly as it will coincide with the airlines 50th anniversary